Unify finding an actor in the internal API

What does this MR do?

This makes all code paths in api/internal/base use the API::Support::GitAccessActor for finding the user.

Having this single path to load a user makes it easier for finding the user to include in logs: gitlab-com/gl-infra/scalability#9 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content